single speaker addressed - multiple speaker playing

1
Hi all,
I'm trying to get a multiple speaker setup with precise timing to work. For some reason, when addressing a single speaker the speaker linked to channel 3 produces the sound as well, just with reduced volume.
I'm using:
- the 8 analog output channels of the recomended M-Audio DELTA 1010LT with native ASIO drivers and Mario's portaudio_x86.dll
- Windows XP SP2
- Matlab R2008b
- 2x Ultron soniccube edition 5.1 (using only the two front and the two surround speakers of each system)

The problem occurs even when addressing a channel that is connected to a different speaker-setup than the channel 3 is connected to. So there should be something wrong with the PC output.

my test-code:

InitializePsychSound(1);
reqlatencyclass = 2;
freq = 96000;
pahandle = PsychPortAudio('Open', 34, 1, reqlatencyclass, freq, 8, 64, [], [0, 1, 2, 3, 4, 5, 6, 7]);

loTone = 0.5 * MakeBeep(200, 2, freq);

% noTone = 0.0 * MakeBeep(100, 2, freq);
noTone = zeros(1,(2*freq+1));

soundArr(:,:,1) = [loTone; noTone; noTone; noTone; noTone; noTone; noTone; noTone];
soundArr(:,:,2) = [noTone; loTone; noTone; noTone; noTone; noTone; noTone; noTone];
soundArr(:,:,3) = [noTone; noTone; loTone; noTone; noTone; noTone; noTone; noTone];
soundArr(:,:,4) = [noTone; noTone; noTone; loTone; noTone; noTone; noTone; noTone];
soundArr(:,:,5) = [noTone; noTone; noTone; noTone; loTone; noTone; noTone; noTone];
soundArr(:,:,6) = [noTone; noTone; noTone; noTone; noTone; loTone; noTone; noTone];
soundArr(:,:,7) = [noTone; noTone; noTone; noTone; noTone; noTone; loTone; noTone];
soundArr(:,:,8) = [noTone; noTone; noTone; noTone; noTone; noTone; noTone; loTone];

for i = 1:8
audMat = soundArr(:,:,i);
PsychPortAudio('FillBuffer', pahandle, audMat);
speaker = i
PsychPortAudio('Start', pahandle, 0, 1);
kbwait
WaitSecs(.2);
PsychPortAudio('Stop', pahandle);
end
PsychPortAudio('Close', pahandle);

Does anybody have an Idea of what I am missing?
Thanks everybody,
Benjamin